….Early warning signs including “callous-unemotional traits” and antisocial behaviour can be identified in childhood and are highly predictive of future psychopathy. Large-scale twin studies have shown that these traits are highly heritable – identical twins, who share 100% of their genes, are much more similar to each other in this trait than fraternal twins, who share only 50% of their genes. In one study, over 80% of the variation in the callous-unemotional trait across the population was due to genetic differences. In contrast, the effect of a shared family environment was almost nil. Psychopathy seems to be a lifelong trait, or combination of traits, which are heavily influenced by genes and hardly at all by social upbringing.

As this shows, the heritability of IQ and of behavioral traits is consistently high, reaching into the 0.8-0.9+ range. This means, out of a group of people, at least 80-90% of the overall differences between them (known as the “variance” in statistical parlance) can be attributed to genetic differences between them. This chart shows that this becomes most evident in adulthood, when genes have been given a chance to fully express themselves. I have summed this up in a neat set of rules:

Various studies have found the heritability of IQ to be between 0.7 and 0.8 in adults and 0.45 in childhood in the United States.[6][18][19] It may seem reasonable to expect that genetic influences on traits like IQ should become less important as one gains experiences with age. However, that the opposite occurs is well documented. Heritability measures in infancy are as low as 0.2, around 0.4 in middle childhood, and as high as 0.8 in adulthood.[7] One proposed explanation is that people with different genes tend to seek out different environments that reinforce the effects of those genes.[6] The brain undergoes morphological changes in development which suggests that age-related physical changes could also contribute to this effect.[20]

In the new study, researchers at King’s College London turned to a cohort of more than 11,000 pairs of both identical and nonidentical twins born in the United Kingdom between 1994 and 1996. Rather than focus solely on IQ, as many previous studies had, the scientists analyzed 83 different traits, which had been reported on questionnaires that the twins, at age 16, and their parents filled out. The traits ranged from measures of health and overall happiness to ratings of how much each teen liked school and how hard they worked. Then, the researchers collected data on how well each individual scored on the General Certificate of Secondary Education (GCSE) exam, an exam that all students in the United Kingdom must take and which is used for admission to advanced classes or colleges.

The team found nine general groups of traits that were all highly hereditary—the identical twins were more likely to share the traits than nonidentical twins—and also correlated with performance on the GCSE. Not only were traits other than intelligence correlated with GCSE scores, but these other traits also explained more than half of the total genetic basis for the test scores.
In all, about 62% of the individual differences in academic achievement—at least when it came to GCSE scores—could be attributed to genetic factors, a number similar to previous studies’ findings, the team reports online today in the Proceedings of the National Academy of Sciences.